A listener detects the time difference, sound pressure difference, and echo, etc., of sound waves reaching right and left ears, and perceives a sound image in that direction. When a head-related transfer function from a sound source to both ears is well tuned with the original sound source in a reproducing sound field, the listener is capable of perceiving a sound image replicating the original sound field in the reproducing sound field.
In addition, sound waves have a unique change in sound pressure level to each frequency until reaching an ear drum through a space, a head, and an ear. The unique change in sound pressure level to the frequency is called a transfer characteristic. When the head-related transfer function is well tuned between the original sound field and the listening sound field, the listener is capable of listening the same tone as that of the original sound by the similar transfer characteristic.
In most cases, however, the head-related transfer function differs between the original sound field and the listening sound field. For example, it is difficult to reproduce the sound field space of an actual or virtual concert hall at a living room. Hence, a positional relationship between a speaker in the listening space and a sound receiving point therein relative to a positional relationship between the sound source in the original sound field space and the sound receiving point therein differs from each other in distance and angle, and thus the head-related transfer function is not tuned well. Hence, the listener perceives the sound image position and the tone that are different from those of the original sound. This is also caused by a difference in number of sound sources between the original sound field space and the listening space. That is, this is also caused by a sound localization method carried out through a surround output scheme by stereo speakers, etc.
Hence, in general, in a recording studio or a mixing studio, sound processing is performed on recorded or artificially created sound signals so as to replicate the sound effect of the original sound under a predetermined listening environment. In the case of, for example, a studio, a mixer engineer expects certain speaker arrangement and sound receiving point, intentionally corrects the time difference and sound pressure difference of sound signals in multiple channels output by respective speakers so as to cause the listener to perceive a sound image replicating the sound source position of original sounds, and changes the sound pressure level for each frequency so as to be tuned with the tone of the original sounds.
International Telecommunication Union-Radio sector (ITU-R) recommends a specific arrangement of 5.1-ch. speakers, etc., and for example, THX defines standards, such as the speaker arrangement in a movie theater, the volume of sound, and the scale of the interior of the movie theater. When the mixer engineer and the listener follow such recommendation and standards, even if the listening environment differs from the original sound field, when the sound signals reach the ear drums of the listener under the listening environment, the sound source position of the original sound and the tone thereof are well replicated.
Although it becomes unnecessary to tune the listening environment with the original sound field, however, a favorable setting of a listening room in accordance with the recommendation and standards is not easy. Hence, respective manufacturers add, to their respective reproducing devices, functions of re-tuning sound signals in accordance with the listening environment produced by the reproducing device, and of replicating the original sound field in a listening room.
For example, there is a known scheme of adding a manual direction tuning function and an equalizer to a reproducing device, enabling a listener to input numerical values for reproducing characteristics, such as the phase characteristic, the frequency characteristic, and the echo characteristic, thereby changing the time difference, sound pressure difference, and frequency characteristic of sound signals in accordance with the input values (see, for example, Patent Document 1).
In addition, there is also a known scheme of mapping the frequency characteristics, etc., of the original sound field beforehand, collecting sound wave signals at a listening position via a microphone, checking the mapping data with the collected data, and tuning the time difference, sound pressure difference, and sound pressure level for each frequency of sound signals for each speaker (see, for example, Patent Document 2).